Rat pheochromocytoma cells (PC12) were established in our laboratory. These cells actively produce catecholamines and contain large amounts of tyrosine hydroxylase and the hydroxylase cofactor, tetrahydrobiopterin. Since it is known that these cells respond to glucocorticoids with an increase in tyrosine hydroxylase activity, we investigated whether there was a concommitment increase in tetrahydrobiopterin levels. Under condition in which there was a 2 to 3 fold increase tyrosine hydroxylase activity, there was no apparent increase in tetrahydrobiopterin (BH4) content of the cells. Furthermore, we could estimate that there was an approximately equimolar ratio of the enzyme to BH4 in control cells but there appeared to be a molar excess of tyrosine hydroxylase subunits in glucocorticoid treated cells.